diff --git a/lib/libc/gen/semctl.c b/lib/libc/gen/semctl.c
index b0ed7235038..96a86269f6e 100644
--- a/lib/libc/gen/semctl.c
+++ b/lib/libc/gen/semctl.c
@@ -1,19 +1,38 @@
 #include <sys/types.h>
 #include <sys/ipc.h>
 #include <sys/sem.h>
+#include <stdarg.h>
+#include <stdlib.h>
 
 #if __STDC__
-int semctl(int semid, int semnum, int cmd, union semun semun)
+int semctl(int semid, int semnum, int cmd, ...)
 #else
-int semctl(semid, int semnum, cmd, semun)
+int semctl(semid, semnum, cmd, va_alist)
 	int semid, semnum;
 	int cmd;
-	union semun semun;
+	va_dcl
 #endif
 {
-#ifdef	__NETBSD_SYSCALLS
-	return (__semctl(semid, semnum, cmd, &semun));
+	va_list ap;
+	union semun semun;
+	union semun *semun_ptr;
+#ifdef __STDC__
+	va_start(ap, cmd);
 #else
-	return (semsys(0, semid, semnum, cmd, &semun));
+	va_start(ap);
+#endif
+	if (cmd == IPC_SET || cmd == IPC_STAT || cmd == GETALL
+	    || cmd == SETVAL || cmd == SETALL) {
+		semun = va_arg(ap, union semun);
+		semun_ptr = &semun;
+	} else {
+		semun_ptr = NULL;
+	}
+	va_end(ap);
+
+#ifdef	__NETBSD_SYSCALLS
+	return (__semctl(semid, semnum, cmd, semun_ptr));
+#else
+	return (semsys(0, semid, semnum, cmd, semun_ptr));
 #endif
 }
diff --git a/sys/sys/sem.h b/sys/sys/sem.h
index ef3086a6b1e..debd5991d38 100644
--- a/sys/sys/sem.h
+++ b/sys/sys/sem.h
@@ -1,4 +1,4 @@
-/* $Id: sem.h,v 1.12 1997/02/22 09:45:51 peter Exp $ */
+/* $Id: sem.h,v 1.13 1997/09/07 05:27:21 bde Exp $ */
 /*	$NetBSD: sem.h,v 1.5 1994/06/29 06:45:15 cgd Exp $	*/
 
 /*
@@ -171,7 +171,7 @@ typedef enum {
 
 __BEGIN_DECLS
 int semsys __P((int, ...));
-int semctl __P((int, int, int, union semun));
+int semctl __P((int, int, int, ...));
 int semget __P((key_t, int, int));
 int semop __P((int, struct sembuf *,unsigned));
 __END_DECLS